Add UnifiedPush Support

[Menrath]

Add support for open and free push notifications via UnifiedPush https://unifiedpush.org/

This can work alongside with Google-Push Notifications. For example see how Element solved this.

This also kind of relates to #263

[hex-m]

We learned recently, that government agencies can access metadata related to push notifications from Apple and Google. This contradicts the privacy statement (german version here) which says

No data is transmitted to other third parties

It's of course not the developers fault, but if it is at all possible, avoiding a dependency to Google services would be most welcome. <3